English

Abstract

Limnologic and water chemistry data from Echo Bay Mineral's Key West and Equinox Resources' Van Stone mines were collected between August, 2000 and September, 2001. Pit lake elevations, areas and depths are 1,318 and 1,050 m; 1.1 and 1.9 ha; and 28 and 31 m, respectively. Depth profiles of temperature, pH, specific conductivity, oxidation-reduction potential (ORP), and dissolved oxygen were collected on a monthly basis during this period. Results indicate that both water bodies are oligotrophic and dimictic, experiencing complete mixing during the spring and fall. Seasonal water temperatures range between near 0° and 22° C and ice covers the lakes approximately 5 months per year. Thermal stratification becomes well developed in both water columns during late summer. The dissolved oxygen profile at Key West is clinograde, ranging from 8-10 mg/l at the surface to 1-2 mg/l at total depth, except during turnover. Van Stone displays an orthograde oxygen profile with relatively high concentrations (>6 mg/l) at lake bottom through out the year. Values for pH tend to fall with depth in both lakes. Waters in Key West and Van Stone pits are slightly alkaline (7-8.5), calcium-sulfate types. TDS concentrations average 628 mg/l at Key West and 386 mg/l at Van Stone; likewise sulfate currently averages about 336 mg/l and 138 mg/l, respectively. Metal concentrations are low in both waters. Contrary to expectation, sulfate concentrations in Key West lake waters have decreased over time, despite abundant sulfide minerals in the pit wall. Also unanticipated, were diminished concentrations of dissolved zinc, arsenic, molybdenum, antimony and selenium concentrations in the oxygen deficient hypolimnion at Key West. The observations and empirical data from Key West and Van Stone pit lakes are useful for, and should be considered, when developing, calibrating, and validating water quality models for circum-neutral mine pit lakes in temperate climates.